One type of field-sequential type color display system comprises a display unit for emitting light rays having wavelengths in a wideband, capable of supplying display information by the light rays of varying wavelengths for respective sub-fields and a variable filter unit for selecting light rays in specific wavelength regions for the respective sub-fields among the light rays having wavelengths in the wideband.
Another type of field-sequential type color display system comprises a light source unit capable of emitting light rays of different wavelengths, and a shutter unit for controlling the light rays emitted by the light source unit on the basis of display information, wherein the light source unit is caused to emit light rays in specific colors for the respective sub-fields while controlling the shutter unit in correspondence thereto.
For a color light source, a fluorescent lamp, or a light emitting diode (LED) has been used. In particular, as a result of the recent development of LEDs emitting blue light, it has become feasible to fabricate the field sequential type color display system by combining LEDs emitting light in the three primary colors.
An example of the field sequential type color display system is shown in FIG. 15.
The field-sequential type color display system is provided with a light source unit 1 composed of a plurality of color light sources which emit light rays of various wavelengths, which can be controlled independently of one another. That is, the light source unit 1 comprises a LED box 3 wherein light emitting diodes (LEDs) 4 for emitting three colors, red, green, and blue, respectively, are arranged as the color light sources, and a diffusion plate 5, and it is driven by a light source driving circuit 8.
The field-sequential type color display system is also provided with a liquid crystal shutter unit 2, operated by the agency of liquid crystal elements, as a shutter unit for controlling the transmittivity of the light rays emitted by the light source unit 1. The liquid crystal shutter unit 2 comprises display segments 6, capable of displaying characters and numbers. And the liquid crystal shutter unit 2 is driven by a shutter control circuit 9.
The shutter control circuit 9 and the light source driving circuit 8 are synchronously controlled by a synchronous circuit 10 so as to be driven in synchronization with each other.
A block diagram of the field-sequential type color display system in FIG. 15 is shown in FIG. 16.
The light source unit 1 consists of a red light source R, a green light source G, and a blue light source B composed of LEDs 4 for three colors, which are energized by a red light source signal Lr, a green light source signal Lg, and a blue light source signal Lb, respectively, supplied from the light source driving circuit 8.
The liquid crystal shutter unit 2 is driven by data signals D and a common signal C respectively supplied from the shutter control circuit 9. Timing pulses of each signal are generated in a synchronous circuit 10 for controlling phases of each light source signal and a liquid crystal shutter driving signal in the same manner.
FIG. 17 is a waveform chart showing waveforms of respective signals in the field sequential type color display system shown in FIG. 16 and optical response characteristic of the liquid crystal shutter unit 2 at the driving voltage of 20V for driving the liquid crystal shutter at room temperature.
In this example, for driving the liquid crystal shutter unit 2 by AC, two fields, f1 and f2, are in use and each of the fields consists of three sub-fields, fR, fG, and fB.
As shown in FIG. 17, the red light source signal Lr turns on only in the sub-field fR, while it turns off in the other sub-fields fG and fB. Similarly, the green light source signal Lg turns on only in the sub-field fG while it turns off in the other sub-fields fB and fR. The blue light source signal Lb turns on only in the sub-field fB while it turns off in the other sub-fields fR and fG.
The voltage of the common signal C supplied to the liquid crystal shutter unit 2 becomes c1 in the field f1 and c2 in the field f2.
When a STN liquid crystal panel in normally white mode is used for the liquid crystal shutter unit 2, a data signal Dw for displaying white is in same phase with the common signal C, and as a voltage is not applied to the liquid crystal panel, the liquid crystal shutter unit 2 is switched to the OFF state, while a data signal Dbl for displaying black is in opposite phase with the common signal C, and as the liquid crystal panel is applied with a driving voltage equivalent to a difference in voltage between the common signal C and the data signal Db1, the liquid crystal shutter unit 2 is switched to the ON state.
A data signal for displaying one of the primary colors is at a voltage such that the shutter is in the transmitting state (OPEN) only in one of the sub-fields corresponding to that color. For example, a data signal Dr for displaying red color is at a voltage such that the shutter is in the transmitting state only in the sub-field fR corresponding to red color while it is in the "closed" state in the sub-fields fG and fB.
A data signal Dg for displaying green color is at a voltage such that the shutter is in the transmitting state only in the sub-field fG corresponding to green color, and a data signal Db for displaying blue color is at a voltage such that the shutter is in the transmitting state only in the sub-field fB corresponding to blue color.
In the case that the LED box 3 is adopted for the light source unit 1, the emission characteristics of the red light source signal Lr, green light source signal Lg, and blue light source signal Lb can be regarded the same as those of respective LEDs since the response time of the respective LEDs, which are semiconductors, is very fast.
Meanwhile, the response time of the liquid crystal panel is slower than that of the LED. Response characteristics at room temperature are shown in FIG. 13 in the case where the STN liquid crystal panel is adopted for the liquid crystal shutter unit 2. The solid line shows the ON response time from the "open" to the "closed" state and the dotted line shows the OFF response time from the "closed" to the "open" state.
The OFF response time is determined by the material of the liquid crystal, the thickness of the liquid crystal cells and the angle through which the liquid crystals are twisted, etc., and it is not dependent on the applied voltage and is always on the order of 1.5 to 3 ms (2 ms in the illustrated example) while the ON response time depends greatly on the driving voltage wherein it is 0.1 ms at a driving voltage of 20V but it reaches 4 ms at a driving voltage of 5V.
In FIG. 17, the span of field f1 is preferably set to 20 ms or less for obtaining good mixing of colors without causing a viewer to perceive flicker, and accordingly, the span of the sub-fields, fR, fG, and fB, respectively, are set to about 5 to 6 ms.
A change from the "closed" to "open" state of the transmittivity Tr of the liquid crystal shutter unit 2 for displaying red is delayed from the data signal Dr for displaying red color by 1.5 to 3.0 ms, equivalent to the OFF response time of the liquid crystal panel. Consequently, the amount of light rays transmitted from the red light source is slightly decreased. Similarly, the transmittivity Tg for displaying green switches to the "open" state behind the data signal Dg for displaying green color by 1.5 to 3.0 ms, and the transmittivity Tb for displaying blue switches to the "open" state behind the data signal Db for displaying blue color by 1.5 to 3.0 ms.
However, as the on response time of the liquid crystal panel from the "open" to the "closed" state is as fast as 0.1 ms at the driving voltage of 20V or more, the transmittivity Tr when displaying red is completely in the "closed" state in the sub-field fG with the result that display in red with good chroma is obtained without mixing of colors caused by the green light source. Similarly, the transmittivity Tg when displaying green will cause no mixing of colors caused by the blue light source, and also the transmittivity Tb when displaying blue will cause no mixing of colors caused by the red light source, thereby displaying respective colors with high chroma.
Data signals for displaying a plurality of the primary colors take a voltage, respectively, such that the shutter is in the transmitting (open) state only in the sub-fields corresponding to each color. For example, a data signal for displaying bluish green takes a voltage such that the shutter is in the transmitting state in the sub-fields fG and fB, corresponding to green and blue, respectively, while in the "closed" state in the sub-field fR. A data signal for displaying purple takes a voltage such that the shutter is in the transmitting state in the sub-fields fB and fR, corresponding to blue and red, respectively. A data signal for displaying yellow takes a voltage such that the shutter is in the transmitting state in the sub-fields fR and fG, corresponding to red and green, respectively.
Such a field-sequential type color display system having the arrangement set forth hereinbefore is characterized in that it can effect multicolor display with a simple construction.
However, with the field-sequential type color display system using STN liquid crystal panels adopted for the liquid crystal shutter unit 2 in normally white display mode, the driving voltage is required to be 20V or more for making the on response time fast, which causes a problem in that a driving IC having a high break down voltage is required, or a boosting circuit is required in the driving circuit, leading to increasing cost of the display system.
FIG. 18 is a waveform chart showing waveforms of respective signals in the field-sequential type color display system shown in FIG. 15 at a driving voltage of 9V for driving the liquid crystal panel at room temperature and optical response characteristic of the liquid crystal shutter.
Waveforms of a common signal C and each of data signals Dr, Dg, Db, Dw and Db1 each supplied to the liquid crystal shutter unit 2 are substantially the same as those of the respective signals shown in FIG. 17, but voltages c1 and c2 of the common signal C are smaller than those of the common signal C shown in FIG. 17 and also voltages d1 and d2 of respective data signals D are smaller than those in FIG. 17.
When the driving voltage is lower, the on response time from the "open" to "closed" state of the STN liquid crystal panel slows down in such a manner as shown in FIG. 13 that the on response time is on the order of 1 to 2 ms at the driving voltage of 9V, namely, it is 10 times or more as slow as at the driving voltage of 20V.
In FIG. 18, the transmittivity Tr when displaying red does not soon switch to the "closed" state even in the sub-field fG, since the on response time from the "open" to the "closed" state slows down, but there is generated a mixing portion Tm where red is mixed with green from the green light source to degrade the chroma of red as purity of color, which is in saturation. Likewise, in the case of the transmittivity Tg when displaying green, there is generated a mixing portion Tm where green is mixed with blue from the blue light source, thereby degrading the chroma of green. Also in the case of the transmittivity Tb when displaying blue, there is generated the mixing portion Tm where blue is mixed with red from the red light source, thereby degrading the chroma of blue.
Accordingly, when the driving voltage is lower, the ON response time from the "open" to the "closed" state of the liquid crystal shutter unit 2 is reduced, and the "closed" state becomes incomplete so that light except the displayed color leaks through, leading to the degradation of the chroma in display segment 6 (FIG. 15) displaying the primary colors of red, green and blue. Accordingly, neither a low-cost driving IC having a low break down voltage nor a low-cost circuit having no boosting circuit can be used, thereby increasing the cost of the color display system.
Further, at low temperatures of 0.degree. C. or lower, the OFF response time slows down, the amount of transmitted light decreases to darken the display color, and the ON response time further slows down, thereby increasing the mixing portion Tm when colors are mixed with those from the other light sources, to degrade chroma, which causes a problem that a range of temperature for operating the color display system is limited in a low temperature zone.